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SAN LEANDRO SHORELINE DEVELOPMENT DRAFT EIR <br />CITY OF SAN LEANDRO <br />AIR QUALITY <br />Central Valley, and small-scale local gradients are often produced along the shorelines of the ocean and <br />bays. The temperature gradient near the ocean is also exaggerated, especially in summer, because of the <br />upwelling of cold water from the ocean bottom along the coast. On summer afternoons, the <br />temperatures at the coast can be 35 degrees Fahrenheit cooler than temperatures 15 to 20 miles inland; <br />at night, this contrast usually decreases to less than 10 degrees Fahrenheit. <br />In the winter, the relationship of minimum and maximum temperatures is reversed. During the daytime <br />the temperature contrast between the coast and inland areas is small, whereas at night the variation in <br />temperature is large. <br />Precipitation <br />The Air Basin is characterized by moderately wet winters and dry summers. Winter rains (November <br />through March) account for about 75 percent of the average annual rainfall. The amount of annual <br />precipitation can vary greatly from one part of the Air Basin to another, even within short distances. In <br />general, total annual rainfall can reach 40 inches in the mountains, but it is often less than 16 inches in <br />sheltered valleys. <br />During rainy periods, ventilation (rapid horizontal movement of air and injection of cleaner air) and <br />vertical mixing (an upward and downward movement of air) are usually high, and thus pollution levels <br />tend to be low (i.e., air pollutants are dispersed more readily into the atmosphere rather than <br />accumulating under stagnant conditions). However, during the winter, frequent dry periods do occur <br />where mixing and ventilation are low and pollutant levels build up. <br />Wind Circulation <br />Low wind speed contributes to the buildup of air pollution because it allows more pollutants to be <br />emitted into the air mass per unit of time. Light winds occur most frequently during periods of low sun <br />(fall and winter, and early morning) and at night. These are also periods when air pollutant emissions from <br />some sources are at their peak, namely, commuter traffic (early morning) and wood -burning appliances <br />(nighttime). The problem can be compounded in valleys when weak flows carry the pollutants up -valley <br />during the day and cold air drainage flows move the air mass down -valley at night. Such restricted <br />movement of trapped air provides little opportunity for ventilation and leads to buildup of pollutants to <br />potentially unhealthful levels. <br />Inversions <br />As described above, an inversion is a layer of warmer air over a layer of cooler air. Inversions significantly <br />affect air quality conditions because they influence the mixing depth (i.e., the vertical depth in the <br />atmosphere available for diluting air contaminants near the ground). There are two types of inversions <br />that occur regularly in the Air Basin. Elevation inversions' are more common in the summer and fall, and <br />s When the air blows over elevated areas, it is heated as it is compressed into the side of the hill/mountain. When that <br />warm air comes over the top, it is warmer than the cooler air of the valley. <br />PLACEWORKS 4.2-3 <br />